The present invention, in some embodiments thereof, relates to a sol-gel process and, more particularly, but not exclusively, to a composition suitable for use in a sol-gel process, e.g., for the fabrication of a layer having a reduced dielectric constant.
In the constant advancement of integrated circuit (IC) technology, it is desired to scale down of IC dimensions, so as to increase speed and density of the chip device. The speed of an electrical signal in an IC relies on the switching time of an individual transistor (transistor gate delay) and the signal propagation time between transistors (Resistance-Capacitance delay, known as the RC delay). As transistor features continue to diminish in size and density continues to increase, the RC delay that was once known to have least impact has now started playing a prominent role in chip performance. RC delay can be reduced by using high conductivity metals in interconnect wiring to lower the resistance (for example, Cu metallization instead of Al metallization), and/or by using materials having a low dielectric constant k in the inter-level dielectric layers to reduce the capacitance. Aside from reducing RC delay, low-k dielectric materials enable consumption of less power and reduce capacitive coupling, also known as crosstalk, between interconnect features.
Different approaches were developed and implemented in industry to decrease the dielectric constant of the interlayer dielectrics and different materials were used for this purpose. The first low-k dielectrics that were applied in industry were fluorinated silica glasses (FSG). FSG are typically not porous. They have a dense structure similar to silicon dioxide layers. SiO2 layers employed as isolating dielectric in IC have tetrahedral basic structure, where each Si atom is bonded to four oxygen atoms, and each oxygen atom to two silicon atoms. The high frequency dielectric constant at practical IC frequencies (up to THz) is about 4 and is associated with the polarizability of the Si—O bonds. To lower this value, the Si—O bonds have been proposed to be partially substituted with less polarizable Si—F bonds. Another option is to dope the silica glass with carbon, by introducing CH3 groups instead of O atoms. Both fluorine and carbon also increase the inter-atomic distances in silica, so that the density therefore also the dielectric constant is decreased. Such materials have characteristic dielectric constants of from about 3.2 to about 3.6.
Another class of low-k dielectrics are hydrogen and methyl silsesquioxanes (HSQ and MHQ) having a structure in which Hydrogen or CH3 radicals are connected to silicon atoms forming cage structures. Silsesquioxanes are organic-inorganic polymers with the presence of cage structures in the form of voids surrounded by eight silicon atoms. SiH and Si—CH3 have lower dielectric constant than Si—O since their polarizability is lower and since their micro-hollows make their volume density relatively small. The dielectric constants of HSQ and MSQ are typically 3.2 and 2.8, respectively.
Another known technique for reduction the value of the dielectric constant k is the use of porous dielectric layers. These layers are provided with pores having sizes which are typically larger than those allowed by the aforementioned cage structure. The typical pores in a porous dielectric layers are from several tens to several hundreds of angstroms. Since the dielectric constant of air is 1, the dielectric constant of a layer decreases with its porosity.
U.S. Pat. No. 5,895,263 discloses a process for forming a dielectric material for IC. The dielectric material is a porous organic polysilica having pore sizes less than 1000 angstroms. The obtained film has a dielectric constant less than 2.8.
U.S. Pat. No. 7,265,062, for example, discloses a process for forming porous silicon oxide-based films using a sol-gel approach utilizing a precursor solution formulation, including a purified non-ionic surfactant and an additive. The additive includes tetraalkylammonium salts and amines. The obtained film has a dielectric constant less than 2.5.